Modern jet aircraft, such as the Boeing 727, employ a plurality of fluid pressure actuators to control the ailerons of the aircraft. Experience has shown that after approximately 10,000 hours of operation, binding of the piston within its supporting bushing is apt to occur, resulting in damage to the actuator body. Such damage is believed to be due to the deterioration of the primary seal which allows the pressurized hydraulic fluid to leak past the primary seal and pressurize the secondary seal. Since the secondary seal is normally held in place by a support bushing, such leakage and pressurization of the secondary seal imposes axial forces on the support bushing. These axial forces are then transferred through the support bushing to the support flanges of the actuator housing. Such support flanges normally have areas of unequal cross section and unequal strength resulting in structural failures to the housing or in some instances resulting in the support bushing being allowed to assume a cocked position to thereby cause the piston to bind or to be seized by the support bushing. This, of course, cannot be tolerated on any commercial or military aircraft. An alternative theory of housing failure or piston binding suggests that an accumulation of manufacturing axial tolerances, plus wear, permits the piston bushing to assume a cocked position relative to the actuator body. It will be appreciated that excessive axial tolerance in such devices can be a problem since the cumulative or net axial tolerance from a manufacturing or assembly process is the sum of the tolerances for all of the components of the assembly. Thus even under the most careful manufacturing or assembly conditions the cumulative axial tolerance will be the sum of the individual tolerances for each of the components in an axial orientation. Excessive cumulative axial tolerance may permit the support bushing to repeatedly impact the support flanges on the housing as the piston assembly is repeatedly cycled. Such repeated impact on the flanges of the housing will cause metal fatigue in the housing or will cause the support bushing to become cocked so as to cause the piston to bind on the cocked surfaces of the bushing.
There is a need therefore, for an improved piston sleeve or bushing for mounting the piston within the actuator body without binding, and which does not rely upon any flange on the actuator body for securing the secondary seal element for the piston.